1) Field of the Invention
The present invention relates to an endovascular prosthesis and, in particular, to a stent/stent graft for treating vascular abnormalities, such as an aneurysm.
2) Description of Related Art
An aortic aneurysm is a weak area in the aorta, the main blood vessel that carries blood from the heart to the rest of the body. A common aneurysm is the abdominal aortic aneurysm (“AAA”), which may be caused by arteriosclerosis. As blood flows through the aorta, the weak vessel wall thins over time and expands like a balloon and can eventually burst if the vessel wall gets too thin. Most commonly, aortic aneurysms occur in the portion of the vessel below the renal artery origins. The aneurysm may be located in the vessels supplying the hips and pelvis, including the iliac arteries.
Once an aneurysm reaches about 5 cm in diameter, it is usually considered necessary to treat to prevent rupture. Below 5 cm, the risk of the aneurysm rupturing is lower than the risk of conventional heart surgery in patients with normal surgical risks. The goal of therapy for aneurysms is to prevent the aorta from rupturing. Once an AAA has ruptured, the chances of survival are low, with 80-90 percent of all ruptured aneurysms resulting in death. These deaths can be avoided if the aneurysm is detected and treated before it ruptures and ideally treated at an early stage (i.e., when the aneurysm is smaller than about 5 cm) with a lower risk procedure.
Aneurysms may be treated with surgery. The surgical procedure for treating AAA involves replacing the affected portion of the aorta with a synthetic graft, usually comprising a tube made out of an elastic material with properties very similar to that of a normal, healthy aorta. However, surgical treatment is complex and may pose additional risks to the patient, especially the elderly.
More recently, instead of performing surgery to repair an aneurysm, vascular surgeons have installed an endovascular prosthesis, (stent/stent graft) delivered to the site of the aneurysm using elongated catheters. The term “stent” refers to a device that is primarily metallic such a balloon or self expanding stent, where as the term “stent graft” refers to a device which comprises a combination of a stent and a natural or polymer fabric or a tubular member, thus the term “stent/stent graft” is used herein to include either configuration, both of which are used to support or line a vessel. Typically, the surgeon will make a small incision in the patient's groin area and then insert into the vasculature, a delivery catheter containing a collapsed, self-expanding or balloon-expandable stent/stent graft to a location bridging the aneurysm, at which point the stent/stent graft is delivered out from the distal end of the delivery catheter and expanded to approximately the normal diameter of the aorta at that location. Over time, the stent/stent graft becomes endothelialized and the space between the outer wall of the stent/stent graft and the aneurysm ultimately fills with clotted blood, which prevents the aneurysm from growing further since the stent/stent graft bypasses (excludes) the aneurysm and prohibits systematic pressure and flow on the weakened segment of the lumen.
Depending on where the aneurysm is in relation to other branch vessels, different design variations may be needed. For example, in treating AAA, the stent/stent graft should be placed so as not to occlude blood flow through the renal arteries which branch off from the abdominal aorta. Moreover, the stent/stent graft should be anchored within the lumen to reduce the incidence of migration, such as by promoting endothelialization or fixation with the lumen. Endoleaks may occur as a result of blood flowing around the stent, which may result in further weakening of the site of the aneurysm.
Furthermore, the size of the delivery catheter may affect the ability of the surgeon to manipulate the catheter within the lumen, often reduced in size due to arteriosclerosis, and may result in trauma to the vascular tissue. Thus, the smaller the delivery catheter, the less trauma to the tissue should occur, and the stent should be more easily and accurately positioned within the lumen. Smaller delivery catheters would also allow a physician access to smaller vessels, so as to more proactively treat aneurysms. Also, smaller aneurysms are typically easier to treat than larger aneurysms (e.g., aneurysms of at least 5 cm in diameter) because smaller aneurysms are more centrally located between the renal arteries and the iliac bifurcation and also because small aneurysms are more symmetrical and usually do not yet include tortuosity, nor involve the iliac arteries.
Conventional stent grafts are typically too bulky to be delivered to treat smaller aneurysms. For example, U.S. Pat. No. 5,800,508 to Goicoechea et al., U.S. Pat. No. 5,916,264 to Von Oepen et al., U.S. Pat. No. 6,110,198 to Fogarty et al., and U.S. Pat. No. 6,709,451 to Noble et al. disclose stent grafts for treating various vascular abnormalities. Although these stent grafts may be radially compressed for delivery, the stent grafts are not configured to be significantly constrained and elongated and may, thus, exhibit a bulkiness that prevents such stent grafts from being delivered to treat smaller aneurysms.
Therefore, there is a need for a stent/stent graft that is capable of being deployed within a variety of lumens for treating aneurysms. Moreover, there is a need for a stent/stent graft that may be easily delivered and adequately anchored within the lumen. There is a need for a stent/stent graft that can be placed in contact with the aneurysm wall, that facilitates tissue in-growth from the vessel wall to the stent/stent graft to strengthen the aneurysm wall and that resists further radial expansion. In addition, there is a need for a stent/stent graft that may be delivered within a lumen that is less traumatic to the vasculature and that may be used to prophylactically treat an aneurysm before becoming large enough to pose a significant health risk to the patient.